This invention relates in general to electrostatography and, more specifically, to a flexible electrostatographic imaging member comprising a conductive polymer.
Flexible electrostatographic imaging members, e.g., belts, are well known in the art. Typical electrostatographic flexible imaging members include, for example, photoreceptors for electrophotographic imaging systems, and electroreceptors or ionographic imaging members for electrographic imaging systems. Both electrophotographic and ionographic imaging members are commonly utilized in either a belt or a drum configuration. When an electrostatographic imaging member is used in a belt form, it may be seamless or seamed. For electrophotographic applications, the imaging members preferably have a belt configuration. These belts often comprise a flexible supporting substrate coated with one or more layers of photoconductive material. The substrates may be inorganic such as electroformed nickel or organic such as a film forming polymer. The photoconductive coatings applied to these belts may be inorganic such as selenium or selenium alloys or organic. The organic photoconductive layers may comprise, for example, single binder layers in which photoconductive particles are dispersed in a film forming binder or multilayers comprising, for example, a charge generating layer and a charge transport layer. Since curling of imaging members often occurs after application of the charge transport layer coating, an anti-curl back coating is applied to the backside of the support substrate, opposite to the electrically active layers, to provide the desired imaging member flatness.
For electrostatographic imaging members in drum configuration, the supporting substrates used are either a rigid metallic or polymeric cylinder. The polymeric cylinder can be optically transparent, translucent, or opaque.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. Nos. 4,265,990 and 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Other electrostatographic imaging devices utilizing an imaging layer overlying a conductive layer include electrographic devices. For flexible electrographic imaging members, the conductive layer is normally sandwiched between a dielectric imaging layer and a supporting flexible substrate. Thus, generally, flexible electrophotographic imaging members generally comprise a flexible recording substrate, a thin electrically conductive layer, and at least one photoconductive layer and electrographic imaging members comprise a conductive layer sandwiched between a dielectric imaging layer and a supporting flexible substrate. Both of these imaging members are species of electrostatographic imaging members.
In order to properly image an electrostatographic imaging member, the conductive layer must be brought into electrical contact with a source of fixed potential elsewhere in the imaging device. This electrical contact must be effective over many thousands of imaging cycles in automatic imaging devices. Since the conductive layer is often a thin vapor deposited metal, long life cannot be achieved with an ordinary electrical contact element that rubs directly against the thin vapor deposited conductive layer. One approach to minimize the wear of the thin conductive layer is to use a grounding brush such as that described in U.S. Pat. No. 4,402,593. However, such an arrangement is generally not suitable for extended runs in copiers, duplicators and printers because wear problems are not entirely eliminated.
Still another approach to improving electrical contact between the thin conductive layer of flexible electrostatographic imaging members and a grounding means is the use of a relatively thick electrically conductive grounding strip layer in contact with the conductive layer and adjacent to one edge of the photoconductive or dielectric imaging layer. Generally the grounding strip layer comprises opaque conductive particles dispersed in a film forming binder. This approach to grounding of the thin conductive layer increases the overall life of the imaging layer because it is more durable than the thin conductive layer. However, such relatively thick ground strip layers are still subject to erosion and contribute to the formation of undesirable "dirt" in high volume imaging devices. Erosion is particularly severe in electrographic imaging systems utilizing metallic grounding brushes or sliding metal contacts or grounding blocks. Moreover mechanical failure is accelerated under high humidity conditions.
Also, in systems utilizing a timing light in combination with a timing aperture in the ground strip layer for controlling various functions of imaging devices, the erosion of the ground strip layer by devices such as stainless steel grounding brushes and sliding metal contacts is frequently so severe that the ground strip layer is worn away and becomes transparent thereby allowing light to pass through the ground strip layer and create false timing signals which in turn can cause the imaging device to prematurely shut down. Moreover, the opaque conductive particles formed during erosion of the grounding strip layer tends to drift and settle on other components of the machine such as the lens system, corotron, other electrical components to adversely affect machine performance. For example, at a relative humidity of 85 percent, the ground strip layer life can be as low as 100,000 to 150,000 cycles in high quality electrophotographic imaging members. Also, due to the rapid erosion of the ground strip layer, the electrical conductivity of the ground strip layer can decline to unacceptable levels during extended cycling.
Micro-crystalline silica particles have been added to ground strip layers to enhance mechanical wear life. Photoreceptors containing this type of ground strip are described in U.S. Pat. No. 4,664,995. The incorporation micro-crystalline silica particles into ground strip layers has produced excellent improvement in wear resistance. However, due to their extreme hardness, concentrations of silica over about 5 percent in ground strip layers has caused ultrasonic welding horns to rapidly wear as the horn is passed over the ground strip layer during photoreceptor seam welding processes. High welding horn wear is undesirable because horn service life is shortened, horn replacement is very costly, and production line down time for horn replacement is increased. An additional problem that is ground strip sensitivity to liquid developer. Exposure to an organic liquid carrier component of a liquid developer causes fatigue ground strip cracking to develop when the ground strip is flexed over small 19 mm diameter belt support roller.
In imaging systems using coherent light radiation to expose a layered member in an image configuration, optical interference occurring within said photosensitive member causes a plywood type of defect in output prints. There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser, is modulated by an input image data signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium. The medium can be, for example, an electrophotographic drum or belt in a xerographic printer, a photosensor CCD array, or a photosensitive film. Certain classes of photosensitive medium which can be characterized as "layered electrophotographic imaging members" have at least a partially transparent photosensitive layer overlying a conductive ground plane. A problem inherent in using these layered electrophotographic imaging members, depending upon their physical characteristics, is an interference effectively created by two dominant reflections of the incident coherent light on the surface of the electrophotographic imaging member; e.g., a first reflection from the top surface of the imaging member and a second reflection from the top surface of the relatively opaque conductive ground plane.
Another shortfall associated with the flexible electrostatographic imaging member belt that has been observed under machine operation conditions is that during electrophotographic imaging and belt cycling processes, the repetitive frictional action of the back side (e.g., the electrically insulative anti-curl back coating) of the imaging belt against the belt supporting rollers is seen to induce electrostatic charge build-up and attract loose toner particles as well as dirt debris to the back side of the belt. These particulate/debris accumulations, when pressed by belt support rollers, produce mechanical protuberances into the imaging belt and causes the development of imaging layer surface cracking. The imaging layer cracking are subsequently manifested as defects in copy print-out.